Happy Birthday Cake Yard Sign Cutout - Coroplast Outdoor Colorful Balloons Happy Birthday Decoration Yard Sign with Metal H-Stake. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. These outdoor Happy Birthday Cake Yard Sign full color direct printed graphics are vertically fluted for use with an H-Stake. Coroplast is typically used for yard, parking, real estate, and election signs. They are waterproof.